Feature request: extension children

[mbouaziz]

If

base class C {
  children =
    | A
    | B
}

is sugar for

base class C
class A extends C
class B extends C

Then I'd like

base class C {
  children =
    | extension A
    | B
}

to be sugar for

base class C
extension class A extends C
class B extends C

And

base class C {
  extension children =
    | A
    | B
}

to be sugar for

base class C
extension class A extends C
extension class B extends C